At present, the prepress technology has been realized in digitalization So that various software for processing computer graphics and images are widely used. In the prepress production, especially in the packaging fabrication, the make-up operation, such as the irregular make-up operation is common. In the make-up operation, a plurality of non-rectangular shapes, figures and/or images are jointly presented on one page and a logical or physical page description file is generated for the subsequent prepress, press and postpress processes.
The input in the make-up process is generally a page description file or a page description flow, such as the PostScript (PS) and the Portable Document Format (PDF). Pages described in these formats are rectangular. Furthermore, the results of these page description files by using a Raster Image Processor (RIP) are also rectangular.
At present, there are two types of page description manners for a same page effect. The page effect shown in FIG. 1 can be taken as an example. According to the first page description manner, only one figure object is described, i.e. the diamond. According to the second page description manner, two figure objects are described. That is, a background rectangle with white color is firstly described, and then the diamond inserted into the background rectangle is described. As shown in FIG. 1, the background rectangle is an envelop rectangle of the diamond.
For the first and second page description manners, the RIP generates a preview of the page described in the received page description file or page description flow according to the following pseudocode.
error = RIP ( PDLFileName, // name of a page description file  default,// use a default gray-scale transferringgraph  “all”, // processing all the pages  ...)// other input and output parameters of theraster image
The default gray-scale transferring graph is shown in FIG. 2. Each component of the output color value is identical to the corresponding component of the input color value when the default gray-scale transferring graph is used. In the raster image process, the gray-scale transferring graph is delivered to the RIP as a parameter.
The RIP starts a raster image process after receiving a page description file or a page description flow. The raster image process is shown in FIG. 3, which includes the following steps.
Step S101 is to determine whether an unprocessed page is contained in the page description file or the page description flow. If yes, the process turns to Step S102; otherwise, the process ends.
In Step S102, the RIP builds a raster image based on page information of an unprocessed page in the received page description file or page description flow and a color space for output, and fills the raster image with the currently defined white color.
The page information comprises the size of the page as well as the resolution required in the raster image process.
Step S103 is to identify the position and the input color value of a figure object in the page, based on the order of filling all the figure objects in the page. The order of filling is defined in the page description file or the page description flow. The input color value is transferred to obtain the output color value.
Since the default gray-scale transferring graph is used presenting the process, each component of the output color value is identical to the corresponding component of the input color value. Thus, the output value of the white color in the figure object will still present the white color.
Step S104 is to fill the raster image with the figure object according to the position and the output color value of the figure object.
Step S105 is to determine whether an unprocessed figure object is contained in the page. If yes, the process returns to Step S103; otherwise, the process returns to Step S101.
In view of the above, the RIP fills the whole raster image with the currently defined white color before filling the page with figure objects. Thus, after the page is processed, the white color filled in the raster image during building the raster image can not be distinguished from the white color in the figure objects in the page filled in the raster image afterwards. In addition, the computer software for processing graphics and images can use only two display manners. According to the first display manner, the colors filled in the raster image are directly displayed. That is, all the areas filled with colors in the raster image are directly displayed and not transparent. According to the second display manner, when a raster image is displayed, the white area in the raster image is transparentized compulsorily. That is, the white color areas in the image are transparent, and displayed in transparency. It can be understood that any other color may also be specified to be transparentized compulsorily. For illustration, the white color is specified in this disclosure. However, in the make-up process, a page sometimes extends into the interior of another page. According to each of the two page description manners, it is possible that the actual result obtained from the RIP could not be correctly displayed, the case of which will be described in detail below.
As to the first display manner, it is assumed that the first page description manner stated above is used for two pages in the make-up operation. The RIP fills the whole raster image with the currently defined white color before inserting figure objects into a page. For example, the figure object described in each of two pages is the diamond as shown in FIG. 1. Where an upper raster image extends into the interior of the diamond of a lower raster image, the displayed make-up result is shown in FIG. 4. That is, the upper raster image is filled with the white color, and it thus covers a part of the diamond in the lower raster image. Actually, in the page description file obtained from the make-up by the RIP, there are only two figure objects, i.e. the diamonds described in the two pages used for the make-up operation. Accordingly, the actual make-up result obtained by the RIP is shown in FIG. 5. It can be seen that the two diamonds do not overlap with each other. The displayed make-up result is not consistent with the actual make-up result obtained by the RIP.
Moreover, for the first display manner, during the make-up operation, since the upper raster image may cover the lower raster image, the user only can determine the positions of the figure objects made-up according to the shapes in the make-up operation, the practical situation of the page and his experience. The displayed result does not present the actual result correctly.
As to the second display manner, it is assumed that the second page description manner stated above is used for two pages in the make-up operation. For example, the figure object described in the each of two pages is the diamond as shown in FIG. 1. The page description file, which is actually obtained from the make-up operation by the RIP, describes the following four figure objects:                1. a white color envelop rectangle in the first page;        2. a diamond in the first page;        3. a white color envelop rectangle in the second page; and        4. a diamond in the second page.        
Where an upper raster image extends into the interior of the diamond of a lower raster image, the displayed make-up result is shown in FIG. 4. That is, the upper raster image covers a portion of the diamond in the lower raster image. Since the white area is transparentized compulsorily for display, the displayed make-up result is shown in FIG. 5. It can be seen that the two diamonds are not covered with each other. Thus, the displayed make-up result is not consistent with the actual make-up result obtained by the RIP.
Furthermore, in the case of using the second display manner, if a white area is contained in the diamond, the white area will also be transparentized compulsorily so that the displayed result does not present the actual result correctly.
In view of the above, in the prior art, when a page is displayed by the RIP, the displayed result of the page dose not present the actual result correctly. Furthermore, the displayed make-up result is not consistent with the actual make-up result obtained by the RIP whatever which display manner of the two manners is used. The white color filled in the raster image during building the raster image can not be distinguished from the white color in figure objects in a page filled in the raster image afterwards.